Foundry molds are nowadays produced from a mixture of sand, resin and if desired a catalyst. The mixing is a step rendered difficult because of the difference of density and of compactness existing between the sand and the resin. Because of this, it is necessary to use high speeds of rotation and long mixing times. One of the problems encountered in the course of mixing is due to the positioning of the point of injection of the resin which is disposed at the periphery of the trough, which renders more difficult the mixing of the resin with the sand during rotation of the screw and involves moreover a rapid clogging of the injection nozzle as shown in FIG. 1.
A partial solution of the problem is provided by the patent FR-A-2.281.786. This apparatus, adapted to mix foundry sand and binders, comprises two mixers which are disposed one above the other on a common vertical rotating shaft and are surrounded by an external envelope. The introduction of the constituents of the mixture is effected by means of a supply device disposed within the shaft and through radial perforations for the outlet of the product from the shaft. Because of its particular construction, this supply apparatus is applicable only to a vertical mixer to avoid on the one hand any obstruction of the radial perforations, and on the other hand a backing up of the material within the shaft between the external walls of the supply device and the internal walls of the shaft. Moreover, this mixer does not permit precise supply of the injected material. Another process for introducing a constituent of a mixture from the interior of a rotating shaft is also described in Japanese patent application JP-A-57136925. This device is adapted to inject a liquid, in particular water, into a mixture. Its very simple conception is not transposable to the injection of pasty or hardenable products.